This invention relates to novel phosphors or luminescent materials that may be used in viewing screens for cathode-ray tubes.
Manganese-activated strontium sulfide phosphors are known to be emitters of visible green light under ultraviolet excitation. These phosphors are also cathodoluminescent. With the improvements in efficiency and the capability of tailoring the emission color as described hereafter, such phosphors could displace presently-used green-emitting phosphors in many applications where photoluminescent or cathodoluminescent phosphors are required.
The novel phosphors consist essentially of strontium sulfide containing about 100 to 3750 ppm (preferably 400 to 600 ppm) manganese and about 50 to 1000 ppm (preferably 100 to 300 ppm) magnesium. The term "ppm" is used herein to mean weight parts of the indicated element per million weight parts strontium sulfide. The effect of incorporating magnesium into manganese-activated strontium sulfide phosphor is to shift the CIE color emission coordinates from about x = 0.323 and y = 0.650 for phosphors with no incorporated magnesium to about 0.370 and y = 0.611 for phosphors with the maximum amount of incorporated magnesium. The luminescence efficiency is improved with increasing amounts of incorporated magnesium within the limits specified by the invention. Reducing the concentration of manganese in the novel phosphors within the limits specified, shifts the CIE coordinates in the direction opposite to that produced by the incorporation of magnesium.
The phosphors are best made with a triple-firing process that involves firing a starting mixture of purified strontium sulfate at about 900.degree. to 1000.degree.C in a hydrogen atmosphere, then refiring the mixture at about 1000.degree. to 1100.degree.C in a hydrogen-sulfide atmosphere, and then again refiring the mixture at about 1100.degree. to 1200.degree.C in a nitrogen atmosphere. The novel phosphors made by this process are efficient emitters under both cathode-ray and ultraviolet excitation.